Summary: In anticipation of programatic needs, we have been developing in vitro methods for measuring neutralization of hepatitis C virus. Hepatitis C virus vaccines have already been proposed. In order to easily evaluate them, an in vitro assay of neutralization will be necessary as opposed to testing in the chimpanzee model. Hepatitis C virus grows poorly in cell culture and its replication is difficult to measure. We are now investigating a new liver cell line developed by Xin Wang and Curt Harris in NCI. We have developed a high sensitivity quantitative real time PCR system for HCV RNA and validated the senstivity and reproducibility of this TaqMan based assay. We have also developed ELISA assays for antibody to the envelope glycoproteins of HCV and for several of the non-structural proteins. These assays will be usefull in vaccine evaluation. At present, we are working on a chimeric mouse model for HCV replication. We can do infectivity studies with this model which should eventually lead to a method for measuring neutralizing antibodies. This has been a long standing goal of our and all HCV virologists.